This invention relates generally to a miniature motor used in audio and video equipment, and automotive electrical components, and more particularly to a miniature motor which can be assembled easily, has high resistance to vibration, impact, etc. and high reliability, and is free from troubles caused by the entry of foreign matter.
FIG. 1 is a partially cross-sectional side view illustrating an example of miniature motor to which this invention is applied. In FIG. 1, reference numeral 31 refers to a case made of a metallic material, such as mild steel, formed into a bottomed hollow tubular shape, and having arc-segment-shaped permanent magnets 32 fixedly fitted to the inner circumferential surface thereof. Numeral 33 refers to a case cover made of a thermoplastic resin material, for example, and formed in such a fashion that the case cover 33 engages with an open end of the case 31. Numeral 34 refers to a rotor consisting of an armature 35 facing the permanent magnets 32 and a commutator, and rotatably supported by bearings 37 and 38 each provided on the case 31 and the case cover 33.
Numeral 39 refers to brush arms made of an electrically conductive material, formed into a strip shape, having at free ends thereof brushes 40 making sliding contact with the commutator 36, and provided inside the case cover 33. Inside the case cover 33 provided are input terminals 41 electrically connected to the brush arms 39 so that power can be supplied from an external d-c power source to the armature 35 via the brush arms 39, the brushes 40 and the commutator 36.
With the aforementioned construction, as current is fed to the armature 35, rotating force is imparted to the armature 35 disposed in a magnetic field formed by the permanent magnets 32 fixedly fitted to the inner circumferential surface of the case 31, causing the rotor 34 to rotate, driving various pieces of equipment connected to the rotor 34.
In a miniature motor having the aforementioned construction, permanent magnets 32 are fixedly fitted inside the case 31 by various means, such as the use of pins, or adhesive. These conventional fixing means inevitably involve increases in the number of components and the time and manhours required for fixedly fitting the permanent magnets 32. This could lead to increased manufacturing cost.
As a means for fixedly fitting the permanent magnets 32 while solving the above-mentioned problems, the use of fixing ribs is known. FIG. 2 is an end view illustrating the essential part of an example of means for fixedly fitting the permanent magnets 32 with fixing ribs. Like parts are indicated by like numerals shown in FIG. 1. In FIG. 2, numeral 42 refers to a fixing rib; four fixing ribs 42, for example, being provided on the inner circumferential surface of the case 31 along the axial direction of the case 31. These fixing ribs 42 can be formed by causing a punch to advance in the axial direction of the case 31, or subjecting the case 31 to partial plastic deformation in the thickness direction. By means of the fixing ribs 42 thus formed and the circular arc surface of the case 31, the permanent magnets 32 can be fixedly fitted to the inner circumferential surface of the case 31.
The miniature motor of the aforementioned construction has an advantage in that the permanent magnet 32 can be fixedly fitted to the inner circumferential surface of the case 31 relatively easily and with less components, but has the following problems.
FIG. 3 is a perspective view illustrating the state where the permanent magnet 32 is fixedly fitted to the case 31. Like parts are indicated by like numerals used in FIG. 2. In the figure, when the arc-segment-shaped permanent magnet 32 is caused to match with and press-fitted to the circular arc surface of the case 31 between a pair of the fixing ribs 42 and 42 in the axial direction (in the direction shown by arrow A) of the case 31. When press-fitting the permanent magnet 32 onto the circular arc surface of the case 31, chips, flakes, etc. are produced from the sliding parts of the permanent magnet 32 and/or the fixing ribs 42 as the edges of the permanent magnet 32 and the fixing ribs 42 rub against each other.
Upon completion of assembly work, therefore, cleaning is needed to remove chips, etc., but these chips, etc. tend to adhere to the surface of the fixing ribs 42, making it difficult to completely remove them. As a result, such foreign matter tends to scatter inside the motor, entering into the gap between the rotor 34 and the permanent magnets 32, as shown in FIG. 1, presenting the risk of a rotor lock and other troubles.
If a gap is provided between the permanent magnet 32 and the fixing rib 42 to reduce the generation of the aforementioned chips, the state of engagement between them tends to become unstable, causing relative movement between the permanent magnet 32 and the fixing rib 42. This could result in lowered characteristics of the miniature motor.